1. Field of the Invention
The present invention is related to a self-raster circuit of a monitor, particularly to a self-raster circuit of a monitor which automatically provides the monitor screen with color rasters when no video signal is inputted from a personal computer.
2. Description of the Prior Art
Rasters are scanning lines obtained by deflecting electron beams projected on a cathode ray tube (CRT) on which video signals are displayed.
FIG. 1 shows a conventional monitor 1 which is connected to a personal computer 3 by a signal cable 2. Accordingly, if the keyboard 4 connected to the computer 3 is manipulated under the power supply to the monitor 1 and computer 3, a video signal corresponding to the input key is displayed on the monitor screen.
FIG. 2 is a block diagram illustrating the construction of the conventional monitor.
Referring to FIG. 2, the conventional monitor comprises a video amplifying section 12 for amplifying a video signal inputted through a signal line 11 connected to a computer main body; a video output section 13 for processing the video signal outputted from the video amplifying section 12 to provide the processed video signal to a CRT 14; a mode controller 15 for detecting a vertical synchronizing signal V.sync and a horizontal synchronizing signal H.sync from the video signal inputted through the signal line 11 to provide a mode control signal; a vertical and horizontal output section 16 for outputting vertical and horizontal drive signals in accordance with the control signal of the mode controller 15; and a flyback transformer 17 for applying to the CRT 14 a high voltage H.V and voltages S, F for controlling the screen and focus of the CRT 14 in response to the horizontal drive signal inputted from the vertical and horizontal output section 16.
The vertical and horizontal output section 16 comprises a vertical drive output circuit 16A and a horizontal drive output circuit 16B. The reference numeral 18, of which the description has been omitted here, denotes a blanking section for outputting a control signal for adjusting the brightness of the picture in compliance to the output signal from the horizontal drive output circuit 16B, and the reference numeral 19 denotes a power supply section for providing powers of various levels required in the internal circuits of the monitor.
FIG. 3 is a schematic circuit diagram of a conventional raster circuit of a monitor in FIG. 2.
According to the conventional raster circuit shown in FIG. 3, a video amplifying section 12 amplifies video signals of R,G,B inputted from the computer by a predetermined amplification factor when the monitor is connected to the computer. The video output section 13 processes and outputs the video signals of red, green, and blue R,G,B from the video amplifying section 12 to the CRT 14 to display the video signals on the CRT 14.
Coils L1 to L3 provided at the input terminals of the video amplifying section 12 reduce the noise included in the video signals R,G,B as well as unnecessary radiation waves EMI. Resistors R1 to R3 connected to the respective output terminals in parallel adjust the impedance of the input video signals. Capacitors C1 to C3 apply only the alternating current components of the input video signals R,G,B to the video amplifying section 12.
The clamp input waveform, as shown in FIG. 4A, which is supplied from the internal oscillating circuit (not illustrated in the drawings) in the monitor through the clamp input terminal Cin, is applied to the base terminal of the transistor Q1. The clamp input waveform as shown in FIG. 4A is converted into a pulse waveform while passing through a resistor R5 and a capacitor C4, as shown in FIG. 4B. Accordingly, the waveform as shown in FIG. 4E appearing at the collector terminal of the transistor Q1 is supplied to the clamp terminal CLAMP of the video amplifying section 12.
A self-testing terminal Sin connected to the video amplifying section 12 is set to be low-leveled when no video signal is inputted into the monitor from the computer. Accordingly, the anode terminal of a diode D1 also becomes low-leveled, thereby turning off the transistor Q1. Since the direct current voltage as shown in FIG. 4C overlaps with the video signals R,G,B inputted from the computer to the video amplifying section 12, the color rasters appearing on the CRT 14 are not varied.
However, when the monitor is disconnected from the computer, and when no video signal is inputted to the monitor from the computer, the self-testing terminal Sin is set to be high-leveled. Accordingly, the clamp signal (refer to FIG. 4A) inputted to the clamp input terminal cin is converted into the clock pulse as shown in FIG. 4B while passing through the resistor R5 and capacitor C4, and inverted while passing through the transistor Q1. The waveform as shown in FIG. 4D is supplied to the anode of the diode D1 and then smoothed to be a DC voltage signal while passing through the capacitor C5, and inputted to the input terminals Rin, Gin, Bin of the video amplifying section 12 through resistors R9, R10, R11. The DC voltage signals of the same level inputted through resistors R9, R10, R11 to the input terminals Rin, Gin, Bin of the video amplifying section 12 cause monochrome rasters corresponding to the DC voltage signals to be displayed on the CRT 14.
However, the conventional self-raster circuit as described above requires a separate testing apparatus to check malfunction of the color processing of the monitor without connecting the monitor to the computer. Further, a separate apparatus is required when a user is to check malfunction of the color processing of the monitor itself. Otherwise, repair of the malfunction should rely on an after-service center.